(The author is a Reuters columnist. The opinions expressed are his own)
By Gerard Wynn
LONDON, Oct 3 (Reuters) - Backing artificial photosynthesis is a risky bet for limited U.S. research funding and with many competing plays in solar energy innovation.
One of the most scientifically ambitious projects to develop clean energy - producing liquid fuel from sunlight, artificial photosynthesis appears futuristic compared with the commercially successful solar PV power.
Of course, governments cannot pick winners every time, and the U.S. spreads the risk by backing other alternative energy sources, but the attraction of copying energy production from plants is clear.
The penalty for picking badly is also clear. The United States backed the semiconductor technology of failed solar photovoltaic (PV) company Solyndra with $528 million in federal loans.
Solyndra filed for bankruptcy, crushed by the dominant incumbent silicon PV technology.
Research is being driven by the Joint Center for Artificial Photosynthesis (JCAP), set up in 2010 as a Department of Energy energy innovation hub with a five-year, $122 million budget to produce a prototype, with no timetable for commercialisation.
Artificial photosynthesis would use the greenhouse gas carbon dioxide (CO2) and combine it with water to produce fuels including sugars or hydrogen for a successor to crude oil.
It would not therefore appear to compete directly with solar PV electricity, which receives a separate tranche of U.S. Department of Energy funds.
But solar PV can also be used to produce hydrogen for transportation - where it is far ahead of artificial photosynthesis - and the electricity it produces could be the future fuel of motor vehicles.
Solar PV is already commercially viable but is still more expensive than fossil fuel power, and is inflexible because it depends on the weather, meaning it requires some form of electricity storage or fossil fuel back-up.
The United States is supporting research to cut the cost of both solar PV and battery storage, the latter through another of five energy innovation hubs, with funding for motor vehicle and electricity storage batteries at $120 million over five years.
Former U.S. energy secretary Steven Chu launched new funding for conventional solar PV through his "SunShot initiative" to make it cost-competitive by 2020.
Through that initiative, the U.S. Department of Energy supports research into a range of PV technologies.
Besides generating electricity, solar PV can be used to split water into hydrogen and oxygen by electrolysis.
The hydrogen can be used in fuel cells or for transport.
Such electrolysis is not yet commercially viable, given it depends on cost-competitive delivery for example through fuel cells and a suitable hydrogen infrastructure.
Artificial photosynthesis is at the laboratory stage.
Researchers at JCAP say they are "inspired" by - rather than aim to directly imitate - natural photosynthesis.
Plants use light to split water into electrons and protons, in turn required for the production of high-energy molecules which help convert carbon dioxide into energy.
Light is harvested by pigments contained in chlorophyll and arranged in convoluted antenna systems whose large surface area increases light absorption.
In artificial photosynthesis, the equivalent to the light-harvesting antennae would be metal semiconductors arranged into high surface area structures using nanotechnology.
Plant enzymes used in natural photosynthesis would be replaced by man-made metal catalysts.
"JCAP's mission is to develop a manufacturable solar-fuels generator, made of Earth-abundant elements, that will use only sunlight, water, and carbon dioxide as inputs and robustly produce fuel from the sun ten times more efficiently than current crops," the research group says in its literature.
"Such an achievement would minimize trade-offs between food and fuel, would allow for installation of the systems in a diverse range of sites and environments, and would provide the direct production of a useful chemical fuel from the sun."
The SunShot initiative aims to achieve its goal of solar PV cost-competitiveness with fossil fuels by 2020.
There is no conflict with artificial photosynthesis provided one is used for transportation and the other for electricity, but there is no reason why either technology should respect such silos.
JCAP describes artificial photosynthesis as a "revolutionary endeavor", and five years makes sense as a limited exploration for development of a prototype.
But an 80-percent drop in solar PV module prices from 2008 through 2012 shows where the competition is. (Editing by Louise Ireland)